The present invention relates to a control valve for regulating a pressure ratio between the pressures p.sub.1 and p.sub.2 in two separate pressure systems.
Control valves of the type referred to herein are known from DIN Standard Sheet 24300 dated March 1966, Sheet 3. Page 7, No. 3.2. which describes what is entitled a proportioning pressure relief valve. The control valve serves to maintain a particular pressure ratio value b between two pressures p.sub.1 and p.sub.2.
Separate pressure systems whose pressures are interdependent are found, e.g. in roll presses of paper machines, especially in wet presses (which extract water from the paper web) and/or in smoothers (which smooth the paper web). In these types of roll presses, at least one of the rolls can be a deflection adjusting roll, e.g. what is known as a floating roll. In such cases, there is a problem in regulating and obtaining desired pressures in two separate pressure systems. One pressure system relates to the effective pressure acting on the pistons of the outer hydraulic cylinder pressing the one roll against the other roll. The other pressure concerns the internal pressure in at least one pressure chamber located in the adjustable deflection roll between the rotating roll shell and the stationary "yoke" passing through the shell. Another possible configuration may involve two counteracting internal pressures of two deflection adjusting rolls whose roll shells are pressed together.
In these types of roll presses comprising two separate pressure systems, the problem is to maintain the ratio between the two pressures p.sub.1 and p.sub.2 constant with a high degree of certainty. In particular, in the event of a defect, for example the sudden failure in the supply of control air, a hydraulic pump failure or a line break, the desired ratio between the two pressures p.sub.1 and p.sub.2 must be restored with the minimum possible time delay. In other words, should one of the two pressures change during a dynamic process, the other pressure should also change immediately to maintain the desired ratio at all times. If this is not attained, there is risk of damaging to the rolls or the felt which runs through the roll press together with a paper web. Or the damage may be to the treated paper web itself. Damage of this nature has frequently occurred in the past because it was not possible to maintain the desired pressure ratio with sufficient accuracy in the dynamic operating conditions of the roll press.
Conventional systems use electrically or pneumatically piloted hydraulic valves to produce a reference pressure in the hydraulic pressure chambers. Even if the regulating valves were to operate without delay in response to dynamic changes in the operating conditions, i.e. to increase or decreases in operating pressures, near instantaneous change in the pressures in the various pressure chambers can not be expected because the machine parts are subjected to elastic deformation and specific quantities of oil need to be pumped through the regulating valves before the deformations are produced. For design reasons, it is impossible to always ensure the same time constants for filling or evacuating the hydraulic working volumes resulting from a deformation in both hydraulic systems during loading and relief operations in this type of conventional control model.
This is evident from examining the hydraulic deflection adjusting roll depicted in FIG. 1 of the German laid-open patent application OS No. 33 28 779, taking standard dimensions as a basis. In the event of a pressure change, e.g. by pressing the two rolls together and increasing the pressure from zero to maximum pressure, on the one hand, the side frames not depicted in the Figure are deformed by 2 mm; on the other hand, the yoke 11 of the roll deflects by 30 mm at its center. The pump for the outer hydraulics producing the pressure in the pressure chambers 32 and 33 of the outer contact pressure hydraulics has a flow volume of 40 liters/min. The pistons have 300 mm diameters. Disregarding leakage and throttle losses, the time to attain full pressure in the outer system is calculated to be 0.42 sec. On the other hand, the displacement volumes during yoke deflection is 140 liters and the delivery output of the pump is 90 liters/min. Hence the time to attain full pressure in the inner system can be calculated to be 93 sec, i.e. more than 200 times greater than in the outer system! It is undesirable to make the outer contact pressure system as sluggish as the inner one because it must be possible to open the press quickly, e.g. in the event of disturbances. Moreover, pressure- and oil-temperature-dependent leakage losses occur in the inner system, depending on the length of time in service (running in of seals). Thus, with conventional means, it is impossible to obtain the sought after simultaneous, constantly precise pressure change in the two pressure systems.
European patent application No. 0 112 625 demonstrates a valve configuration for a system referred to herein. The configuration includes a pneumatically controlled valve piston to control the pressure p.sub.1 in a first hydraulic system, and a control valve in the form of a floating piston to control the pressure p.sub.2 in a second hydraulic system. The pressure p.sub.2 is correlated to the pressure p.sub.1 in a first hydraulic system. Under stationary operating conditions, the control valve is capable of producing a control characteristic between the two pressures according to the equation p.sub.2 =b.multidot.p.sub.1. However, it is impossible with that valve to correlate more than two pressures, as is occasionally desired. A more serious situation is that, in the event of failure, the desired pressure ratio p.sub.2 /p.sub.1 can not be maintained with the known valve. Hence, according to another European patent application No. 0 109 220, a further safety valve is provided between the two hydraulic systems having p.sub.1 and p.sub.2. The latter valve has the property that, should the pressure in one of the two systems be greater by a specific amount than corresponds to the control characteristic, a valve opens and relieves the overpressure. However, since spring force must be overcome to open the valve and since frictional losses occur in the lines and in valve bodies, the device is also incapable of achieving a precise control behavior.
An object of the present invention is to solve the problem of providing a control valve for regulating a specific ratio between the pressures of two separate pressure systems. The valve should maintain a desired pressure ratio between two pressure systems with greater accuracy than previously known, both under stationary and dynamic operating conditions. Simultaneously, the pressure systems should have the simplest possible configuration in order to lower manufacturing costs and avoid errors when adjusting the machines. Finally, it shall be possible as required to alter the ratio between the two pressures in the simplest manner.
According to the present invention the piston and the housing of the control valve form a total of two overflow gaps. The parts are spatially separated in such a manner that the fluids of the two pressure systems remain completely separated from each other. As required, the low-pressure chambers, into which the pressurized fluid flows after passing the overflow gaps, may also be separated from each other. However, since the same pressurized fluid is frequently used in the two pressure systems, and since the two low-pressure chambers are frequenctly subjected to the same pressure, e.g. atmospheric pressure, only one low-pressure chamber common to the two overflow gaps may be provided.
In the control valve according to the invention, the two piston end faces subjected to the pressures of the two pressure systems can be of equal size as required if the ratio between the two pressures must be constantly equal to 1. However, as a rule, the pressure ratio deviates from 1 and therefore the end faces have unequal sizes.
All the embodiments of the control valve according to the invention have the following objective. Whereas regulation of the second pressure p.sub.2 occurs at one overflow gap during the normal state of equilibrium by a continuous flow of a small quantity of fluid from the second pressure chamber into the low-pressure chamber as in control valves of the art, in the additional overflow gap provided according to the invention there is no regulation process; i.e. the piston maintains the additional overflow gap generally closed. A pressure regulation process only occurs at the additional overflow gap if the second pressure p.sub.2, (e.g. in the event of any disturbance) undershoots the desired value. In this case, provision is also made for the first pressure p.sub.1 to decrease simultaneously with the second pressure p.sub.2, thus maintaining constant the desired pressure ratio. The inventor perceived that the omission of the additional function in control valves of the art was the cause of the previously described difficulties.
Another objective is that the control valve according to the invention take into account not only the pressure p.sub.1 prevailing, for example, in the pressure chamber of the deflection adjusting roll, but also a counterpressure, for example, the variable counterpressure p.sub.3 (changing with oil viscosity and gravity conditions) acing on the other side of the yoke of the deflection adjusting roll.
Another objective is that the pressure difference p.sub.1 -p.sub.3 follow an exactly linear progression to pressure p.sub.2.
In the case of a control valve according to the invention, the additional force F5 can contribute towards the fact that, for example, the net weight of a vertically pressing roll can be taken into consideration during the pressure regulation process. Thus, the control valve is not only capable of generating a control characteristic according to the equation p.sub.2 =b.multidot.p.sub.1, but can also generate a control characteristic according to the equation p.sub.2 =a+b.multidot.p.sub.1, where the differential value a is determined by the additional force F5. If, as mentioned above, the counterpressure p.sub.3 must also be taken into consideration, the equation for the control characteristic will be p.sub.2 =a+b.multidot.(p.sub.1 -p.sub.3).
In principle, at least one of the control edges of the pistons, as is known, could be formed by a circumferential slot machined into the surface of the piston skirt. However, the known configuration will be preferred since it is simpler and space-saving and it offers the possibility of altering as required the value b of the control characteristic by using simple means, which are described herein.
With the guide bushing mentioned herein, the objective is achieved that the piston can be displaced in the housing with almost complete freedom of resistance. This largely allows the control valve to operate with the desired accuracy.
In accordance with another objective, a further counterpressure acting against the second pressure p.sub.2 is taken into consideration, the equation thus becoming: p.sub.2 -p.sub.4 =a+b.multidot.(p.sub.1 -p.sub.3). Means are provided for the control process to remain unaffected in the event of changes in the counterpressure p.sub.4.
Certain features presented herein are especially recommended for those parts of the control valve forming the additional overflow gap according to the invention. Since fluid actually flows through the additional overflow gap during relatively seldom occasions, there is the danger that a contaminating particle may lodge itself between the piston and the housing bore, thus causing friction. The taper provided firstly allows the constant flow of a very small quantity of fluid through the additional overflow gap (without the occurrence of a regulation process there), thus removing any contaminating particle present, and secondly a sufficiently high flow resistance is present in order to allow the draining away of a low quantity of fluid.